Heretofore, a heat-sensitive sheet for stencil printing was not necessarily satisfactory in definition or sharpness of printed images, particularly in evenness of its solid parts. While various causes can be adduced for such circumstance, a condition ascribable to the fibers which constitute the support can be mentioned as one of the causes.
That is, a tissue paper (thin paper) which is conventionally used as the porous support in a stencil sheet and composed of natural fibers has defects as follows:
Passing of ink is liable to become uneven since diameter of the fibers is comparatively large and uneven, and cross section of the fibers is flat. Particularly, passing of ink is obstructed by the fibers located at directly under perforated portions to cause fading (or blurring) of printed letters, and PA1 Smoothness of the surface of a film laminated on the support is deteriorated by the thick fibers, and contact of the film with a thermal head at the time of perforations is poor to often causes deficient perforations, thus, voids are formed in solid printing.
In order to improve these defects, such countermeasures have been proposed that a paper prepared by using a blend of natural (cellulosic) fibers and synthetic fibers such as polyester fibers through a wet papermaking process or a non-woven fabric is used instead of the above mentioned tissue paper composed only of natural fibers to make the fibers in the porous support fine on average or to reduce the basis weight of the paper or fabric (Laid-open Japanese Patent Publication Nos. Sho 59-2896, Sho 59-16793, and Hei 2-67197).
Definition of images is improved by thinning the diameter of fibers in the porous support or reducing the basis weight of the paper or fabric. In this case, however, there occur such problems that runnability of the sheet is lowered to cause jamming in the printing apparatuses during feeding, and that wrinkles occur on the sheet when an unperforated or perforated sheet is wound around and loaded on a printing drum, and the wrinkles degrade the quality of printing. Besides, there is a defect that the sheet is elongated (elongation at printing) or wrinkles occur on the sheet (wrinkles at printing) and thus reproducibility of a manuscript in printed papers is lowered when a number of papers are printed.
Further, in order to improve these defects, a stencil sheet having a specific tensile strength and flexural rigidity, that is, a specified tenacity and stiffness (Laid-open Japanese Patent Publication No. Hei 8-67080), and another stencil sheet having a specific wet elongation when the sheets is stretched under a certain load (Laid-open Japanese Patent Publication No. Hei 5-104875) have been proposed.
However, even when the sheet has the strength and rigidity (stiffness) described above, the effect of preventing the wrinkle occurrence when the sheet is wound around a printing drum is low, and the sheet is not thoroughly satisfied even as to the wrinkle occurrence at printing.